The book of the Revelation was written in Greek. The Greeks had no number figures; they used alphabets to represent numbers. The modern arithmetic number figures and number system were developed from Hindu-Arabic roots in early 13th century AD.
In the old Greek number system, a number like 666 could be written in a thousand different ways! Any or all of these could be used by the Antichrist! Hence “Let him that has understanding count (calculate/figure out) the number of the beast” (Rev 13:18).
When calculated/counted/figured in the Arabic figure number system, the number is “666”, but in the Roman numeral system it is “MCLXVI”, which incidentally contains all the numeric letters that make up the Roman numeral system arranged in order from the maximum to the minimum!